Volunteers, under the direction of scientist Stuart Weiss, released 400 Bay Checkerspot caterpillars into Edgewood County Park and Natural Preserve in Redwood City on Thursday.

The park's butterfly population became extinct in 2003. The Bay Checkerspot butterfly is on the federal threatened list of endangered species.

The caterpillar release was done as part of a celebration commemorating the 20th anniversary of the park becoming a nature preserve. For more about Edgewood County Park, visit www.friendsofedgewood.org.